The structure and action of multisubunit enzymes and proteins are being studied by methods of x-ray diffraction and biochemical techniques. The three proteins currently under study are: (1) Glutamine synthetase from E. coli and Salmonella tiphimurium, a highly regulated enzyme that controls the assimilation and metabolism of nitrogen-containing compounds. (2) Ribulose bisphosphate carboxylase/oxygenase (RuBisCO), said to be the world's most abundant protein, which catalyzes the initial reaction both of photosynthesis and photorespiration, and (3) Melittin from bee venom, a membrane-related protein. A structure at 2.0 A resolution has been obtained for this protein, and the mechanism of its toxicity is being interpreted in terms of the structure.